In case of MCCS needed data cannot be shared among nodes in a cluster, namely NAS or DAS, may not use latest data after failover. In this case, Mirror disk is used when there is no external shared disk, and is replicated between two nodes by using replication component. MCCS provides replication function of DataKeeper from SIOS corporation (OEM Product) and Mirror Disk Agent manages this replicated data set.

It functions properly according to the state and role of mirror disk. Actions such as state and role of mirror disk are all recorded in the system event log, and this information will be sent to MCCS through event log monitor module.
MCCS Event Module is activated when MCCS service starts.
Data replication software operates by creating mirror set for volumes between two nodes.
The primary server has source volume and the secondary server has target volume which is an exact replica of source volume.
Client is only available to read/write in the source volume, changed block of the volume is replicated to the target volume through the TCP/IP network connection. At this point, target volume is in lock state and read/write is not allowed. This is to ensure data integrity by preventing the use of target volume.

Table of Contents

[Figure] Mirroring Configuration

Replication Mode

Replication option employs both asynchronous and synchronous mirroring schemes. Understanding the advantages and disadvantages between synchronous and asynchronous mirroring is essential to the correct operation of this.

Async Mode

With asynchronous mirroring, each write is captured and a copy of this is made. That copy is queued to be transmitted to the target system as soon as the network will allow it. Meanwhile, the original write request is committed to the underlying storage device and control is immediately returned to the application that initiated the write. At any given time, there may be write transactions waiting in the queue to be sent to the target machine. But it is important to understand that these writes reach the target volume in time order, so the integrity of the data on the target volume is always a valid snapshot of the source volume at some point in time. Should the source system fail, it is possible that the target system did not receive all of the writes that were queued up, but the data that has made it to the target volume is valid and usable.

Sync Mode

With synchronous mirroring, each write is captured and transmitted to the target system to be written on the target volume at the same time that the write is committed to the underlying storage device on the source system. Once both the local and target writes are complete, the write request is acknowledged as complete and control is returned to the application that initiated the write. With synchronous mirroring, each write is intercepted and transmitted to the target system to be written on the target volume at the same time that the write is committed to the underlying storage device on the source system. Once both the local and target writes are complete, the write request is acknowledged as complete and control is returned to the application that initiated the write.

Adding

Add the mirror disk resource to a group.

When add a resource from a group name, select a group → right click → 'Add Resource'. Or select Edit(E) from the main menu bar → select 'Add Resource'. Or select 'Add Resource' icon from the tool bar.
Select 'MirrorDisk' from Resource Type lists and click 'Next' button. When add a resource from the mirror disk resource type name, this step will be skipped.
Select the mirror volume and MCCS will renew the information.
If you skip enter resource name, this will given 'Mirror_value' which value is selected driver letter.(Ex: Mirror_G)

The drive characters to be used as the mirror disk should be same on the both servers.
[Drive characters in the SAN environment (shared disk) are also displayed in the list. So, make sure you select the accurate drive characters.]
Data will be copied to the target node from the source node.
Be careful when you select the drive character where data exists.
If you try to add the existing mirror volume, <mirrored> will be added to the volume letter.

[Figure] Mirror Disk Resource Added
As 'Additional Settings', Synchronized Mode and Check Disk option is available.

<Synchronized Mode>
Once selected, the synch mode cannot be reversed. So, be careful.
Sync mode : Response is slow but there is no data loss involved.
Async mode : Relatively faster than the synch mode but it can suffer from data loss.

<Check Disk Option>
Set the check disk command option. For more details on the function, refer to MSDN.
[Figure] Defining mirror type and check disk option
When click OK button, following popup message appear.
Confirm selected server as source.
If you select wrong sever, clock cancel button.

[Figure] Alert popup message after click OK button
Click the 'Finish' button to add the mirror disk resource.
You can immediately check the result in the management web console.

Deleting

Select resource type → right click → delete resource.

Click "Delete resource" and a confirming message about deleting resource will appear.

[Figure] check resource view
Click "OK" and a confirming message about deleting redundancy program of mirror configuration will.

[Figure] Delete physical mirror disk view
Click "OK" and redundancy program is deleted. Click "Cancle" and only mirror disk is deleted.
The deleted resource will immediately disappear from the management web console.

Status

The following table explains the status switching of the MCCS resource caused by a user's command and the status.
The command assumes that it is generated by a user.

Mirror disk agent: Manages the mirror disk. Copying program should be installed.

Status	Agent command	Description
Online In this status, you can access the source volume and perform a writing test properly.	Offline	Regardless of the mirror role, the LOCKVOLUME command is used to lock the volume. If the mirror volume is not defined, the LOCKVOLUME command is ignored and regarded as a failure.
	Monitoring	The monitoring process is also determined by the mirror volume status and roles. The kernel driver of copying program records an event in the system event log when the mirror volume status is changed. The MCCS event monitor is registered as an event receiver when starting the MCCS service. When a system event occurs, it will be automatically informed and it will detect the mirror volume status and role changes. Once the status and the role are determined, a disk writing test is performed to finalize the resource status. <Source volume> If the writing test is successful, it is regarded as the online state. <Target volume> 1. If the writing test failed on the disk, it is regarded as the offline state. 2. If a user unlocks the target volume while the mirror disk is online, an agent will automatically lock it again. Thus, the target volume of the node with the online mirror disk cannot be unlocked. 3. If the mirror volume is not defined, it is regarded as the offline state.
Offline Except for the online and trouble states, it is always offline.	Online	Type of operation is determined by the role of the mirror volume at the start node. <Source volume> 1. Allow them to unlock and write to the volume. 2. Unlocking is done by 'UNLOCKVOLUME' of the copying program. The write function is executed after changing the MountReadOnly in registry value of the copying program to 0. After writing is available, the value is changed back to 1. <Target volume> 1. An attempt to go online at the node where the mirror role is the target failed if the mirror status is not "Mirroring". 2. If the mirror volume is fully synchronized, you can change the mirror role. Such a process is determined by the node status (A). 3. If you cannot connect to mirror driver of node A, use the DELETEMIRROR command to delete the existing mirror volume. After that, switch the target and the source, and use the CREATEMIRROR command to create the reverse mirror volume. When creating the mirror, if you set the option to 0x1, they are not all synchronized. 4. If you cannot connect the mirror driver of node A, use the DELETELOCALMIRRORONLY command to delete the mirror configuration of node B only. After that, use the CREATEMIRROR command to configure the reverse mirror volume. Now, node A does not respond. The 0x2 option is added so that it does not wait until the target mirror volume is created. If the final volume is locked, unlock it and change the MountReadOnly option to 0. After writing, change it back to 1. If the mirror volume is not defined, it is processed as a failure without any operation.
	Monitoring	Refer to the description of monitoring as above.
Trouble If a writing test failed online, or an attempt to go online is failed, the trouble state is displayed. *Failover deactivation state When a mirror network communication failure, target node or target disk failure occurs, the mirror state is changed from 'MIRRORING' to other, such as 'MIRROR_PAUSED'. If the mirror status is not 'MIRRORING', a failover will take place and it can cause data losses or damages. To prevent this problem, an agent deactivates the failover mode if the mirror status is not "MIRRORING". If the failover mode is deactivated, no failover will take place even if manual failover or troubles occur. If mirror network communication failures are recovered, and communication resumes, it will automatically perform RESYNC. When the mirror status is changed to MIRRORING, the failover mode will be also automatically activated.	Online	Refer to the above online command.
	Offline	Regardless of the mirror role, the LOCKVOLUME command is used to lock the volume. If the mirror volume is not defined, the LOCKVOLUME command is ignored and regarded as a failure.

Attribute

Show the unique attribute of resource.

Attribute	Desctiption	Type	Default	Edit	Note
AutoResolveSplitBrain	If it is set to 'true', it will automatically resolve the split brain issues.	BOOLEAN	true	O
ChkDsk	When the volume is mounted, according to the 'ChkdskArg', disk is tested for errors. If it is set to true, and if dirty bits are set to the online volume, the ChkdskArg option will be used to run checkdisk. Before running checkdisk, the mirror copying will be automatically stopped and resynchronization will be performed after finished..	BOOLEAN	false	O
ChkdskArg	When the Chkdsk option is set to true, the check disk command option is activated.	STRING	/F	O
DirtyBit	It means that dirty bits are set to the target mirror volume. In general, if you need to check the integrity of the target volume, the system will set this.	BOOLEAN	false	X
DomainMemberServer	If the node to configure the mirror resource is a member server of the Windows domain, you must set it to "true". Now, the Windows service account to start the DK service should be set as Administrator for the local node.	BOOLEAN	false	O
DriveLetter	The mirror volume is assigned to the drive character. It should be configured to obtain the same value at the source and the target systems.	STRING		X
MirrorIP	The IP address used to copy the volume.	STRING		X
MirrorRole	The mirror role of the mirror volume. It is renewed from an event log.	STRING		X
MirrorState	The mirror status of the mirror volume. It is renewed from an event log.	STRING		X
MirrorTimeout	This is the time limit for mirror role or status updates in the process of volume failovers or switching. If the status or role is not renewed within this time period, a time out error will occur.	INTEGER	120 sec	O
MirrorType	Determine the mirror synchronization mode. Set it to A or S. ( A - async mode, S - sync mode) * If it is set to asynch mode, data loss can occur.	STRING	A	X
Password	It is the password for the designated user account. Unless you set it otherwise before, now, enter the password for the 'Administrator' account.	PASSWORD		O
ResyncNeed	It represents the total sum of the blocks requiring resynchronization.	INTEGER	0	X
ResyncTotal	It is the total sum of the resynchronization blocks.	INTEGER	0	X
SourceProrityLevel	It indicate SourceRole Level of each node in a cluster. Higher value of node take Source Role.	STRING		X
TotalSpace	It represents the total size of the target mirror volume.	INTEGER		X
UsedSpace	It represents the available size of the target mirror volume.	INTEGER		X
User	The user account to run the mirror related commands. Unless you set it otherwise before, now, the 'Administrator' account will be used.	STRING	Administrator	O

The attribute value is entered to register the resource, and you can change or check it in the 'Specified Attribute'.

7.6 Mirror Disk